Cyclic viscoelastoplasticity of polypropylene: Effects of crystalline structure

Observations are reported on two grades of polypropylene in tensile tests with various strain rates, relaxation tests with various strains, and cyclic tests with a stress-controlled program (ratcheting). Experiments are performed on isotactic polypropylene (iPP) manufactured by the Ziegler-Natta catalysis and metallocene-catalyzed polypropylene (mPP). The time- and rate-dependent behaviors of iPP and mPP in tensile tests and relaxation tests are quite similar, whereas their responses in cyclic tests differ pronouncedly: The number of cycles necessary for mPP to reach a required ratcheting strain exceeds that for iPP by an order of magnitude. To rationalize these observations, a constitutive model is developed in cyclic viscoelastoplasticity of semicrystalline polymers, and its adjustable parameters are found by fitting the experimental data. Slowing down of growth of ratcheting strain in mPP is attributed to the presence of small crystalline domains in amorphous regions that act as physical cross-links. The effect of the strain rate on the number of cycles to failure is studied numerically.